The present invention relates to a method for functional monitoring of mechanical paper shredders in which the sheet material to be destroyed is guided between rotating and meshing cutter blocks.
European print 0 184 786 B1 discloses a mechanical paper shredder for destroying sheet material, in particular bank notes no longer fit for circulation. The bank notes to be destroyed are fed to a cutting unit by means of a transport system. The cutting unit comprises a pair of meshing cutter blocks rotatably mounted in a housing. The cutter blocks have a plurality of cutter disks separated by distance plates. The width of the cutter disks is slightly smaller than that of the distance plates so that the cutter disks of each block protrude into the spaces in the other block without touching it. On the other hand the distance between two meshing cutter disks is clearly smaller than the thickness of the sheet material to be destroyed.
A bank note fed to the cutting unit is cut up in the longitudinal direction by the pair of meshing rotating cutter blocks. Furthermore the bank note is squeezed due to the distances between two meshing cutter disks and cut or torn up in the transverse direction by notches in the cutter disks. Altogether the bank note is cut into small rectangular shreds. Due to the tearing effect in the transverse direction the paper structure is irreversibly destroyed. The combination of such shreds with intent to defraud can thus be excluded.
After the destruction operation the shreds are collected in a vessel. To support the collecting operation the collecting vessel is connected to an air suction system which can produce a directed air stream in the cutting unit. The air stream carries the bank note shreds resulting from the cutting operation into the collecting vessel, churning them and simultaneous cooling the cutting unit. There is thus no danger of a jam being caused by bank note shreds since any shreds sticking to the cutter blocks are carried off by the air stream. Also, the noncontact rotation of the meshing cutter blocks and the cooling produced by the air stream permit continuous operation of the cutting unit.
During operation of such known cutting units it has now been ascertained that soiling arises on the cutter blocks that cannot be eliminated even by a directed air stream. The soiling presumably comes primarily from the bank notes squeezed between the cutter disks during the cutting operation. In the phase in which the bank note strips are torn in the transverse direction they are squeezed between the cutter disks so that the cutting unit no longer works in noncontact fashion. Instead, the bank notes to be cut rub on the cutter disks on both sides, thereby increasing the heat buildup locally in the otherwise frictionless cutting unit and favoring the deposition of greasy and inky paper particles of soiled bank notes. This cannot be prevented by the directed air stream, especially since the effect of the air stream between the cutter disks is negligible during the cutting operation.
Particularly when very soiled bank notes are destroyed, the spaces in the rotating, meshing cutter blocks are loaded in the course of time to such an extent that the function of the paper shredder can be jeopardized. It has been shown that, unless countermeasures are taken, the run of very soiled cutter blocks can result in damage to the drive motor or tearing of the driving belt for the cutter blocks.